It is known that the polyurethane systems used for the production of moldings show strong adhesion to the mold materials used, preferably thermally highly conductive materials, such as metals. In the demolding of the polyurethane moldings, release agents, which are applied to the mold walls coming into contact with polyurethanes and/or the polyurethane reaction mixture, are therefore required.
Such release agents consist of dispersions or emulsions of waxes, soaps, oils and/or silicones in solvents, such as hydrocarbons or water.
After application of the release agent to the mold, the solvent evaporates and the non-volatile substances having release activity form a thin release film which is intended to ensure that the polyurethane molding can be easily removed from the mold after production.
In addition to the actually required release effect, the release agent also performs further functions; thus, it also greatly influences the surface of the polyurethane molding, which should be fine-pored or smooth and uniform, inter alia also for ensuring good coverability of the finished shaped articles with textiles or leather.
In order to reduce the environmental pollution with organic material, there is considerable interest in water-based release agents that are free of volatile organic material. However, compared with classical release agents containing organic solvents, the aqueous release agents on the market have the disadvantage that, after evaporation of a major part of the water, a thin water film remains in the mold. This water film does not volatilize at the usual mold temperatures of from 45° to 80° C., preferably from 50° to 75° C., and undergoes with the isocyanate compounds of the polyurethane system reactions that lead to very rigid polyurea compounds. The mold surfaces are adversely affected as a result. The mold surfaces acquire a so-called build-up which has to be removed by complicated cleaning.
Typical examples of aqueous release agents having a good demolding effect are described, for example, in DE-A-37 42 370 or DE-A-40 20 036. These release agents contain, as a substance having release activity, unsaturated oligomeric or polymeric hydrocarbons having molecular weights of at least 500 and iodine numbers of at least 60, particularly preferably liquid polybutadiene having molecular weights of about 3000 and iodine numbers of about 450.
Particularly disadvantageous in the case of these release agents are the strong discolorations, which are caused by oxidation of the double bonds.
A further problem of the release agents cited above is the frequent occurrence of allergies among employees who work with these release agents or the shaped articles produced with their aid.
In addition to the above-described problem of the polyurea buildup on the mold surfaces, however, these release agents also have the further disadvantage that the viscous oily release substance greatly soils the working environment. A tacky greasy film which is difficult to remove forms on the machines and plants; even with considerable care, soiling of the environment cannot be avoided with certainty, so that a slippery greasy film, which is difficult to remove and constitutes an unacceptable potential hazard for the employees forms, for example, on the floors.
This greasy film can also further react owing to the double bonds present in the unsaturated polymeric hydrocarbons, for example to give resinous, tacky layers which can scarcely be removed with conventional cleaning agents.
U.S. Pat. No. 4,936,917 mentions, as substance having release activity, inter alia glyceryl fatty acid esters, for example, hydrogenated glyceryl tristearate, which are solid at the filling temperature of the mold. However, these solid substances having release activity cannot eliminate the above-described problems of the polyurea buildup in the mold, caused by aqueous release agents.
In view of the above, there is an ongoing need for providing aqueous mold release agents that are free of organic solvents, which have a good release effect, and which advantageously influence the surfaces of the polyurethane moldings, i.e., leave them fine-pored, uniform and smooth, leave behind no polyurea buildup on the mold surfaces, do not soil the working environment with a greasy film and have a low allergy potential.